The present invention relates to a fuel injector according to the species of the main claim.
From German Patent 33 14 899 A1, an electromagnetically actuatable fuel injector is already known, in which, for the electromagnetic actuation, an armature cooperates with an electrically excitable solenoid coil, and the stroke of the armature is transmitted to a valve-closure member via a valve needle. The valve-closure member cooperates with a valve seat surface to form a sealing seat. The armature is not fixedly mounted on the valve needle, but is arranged thereon so as to be axially movable. A first resetting spring acts upon the valve needle in the closing direction and thus holds the fuel injector closed in the currentless, non-excited state of the solenoid coil. The armature is acted upon by a second resetting spring in the stroke direction such that the armature in the neutral position contacts a first limit stop provided on the valve needle. When the solenoid coil is excited, the armature is pulled in the stroke direction, carrying the valve needle along with it by the first limit stop. When the current exciting the solenoid coil is switched off, the valve needle is accelerated by the first resetting spring so as to move into its closing position, the valve needle carrying the armature along with it by the limit stop described above. As soon as the valve-closure member meets the valve seat, the closing motion of the valve needle is abruptly ended. The motion of the armature, which is not fixedly joined to the valve needle, continues in the stroke direction and is absorbed by the second resetting spring, i.e., the armature swings through against the second resetting spring, having essentially a smaller spring constant than the first resetting spring. Finally, the second resetting spring accelerates the armature once again in the stroke direction.
If the armature strikes the limit stop of the valve needle, this can lead to the valve-closure member, which is connected to the valve needle, lifting off once again for a short time from the valve seat, thus leading to a short-term opening of the fuel injector. Therefore, in the fuel injector known from German Patent 33 14 899 A1, the debouncing is imperfect. In addition, both in a conventional fuel injector, in which the armature is fixedly joined to the valve needle, as well as in the fuel injector known from German Patent 33 14 899 A1, it is disadvantageous that the opening stroke of the valve needle commences as soon as the magnetic force exerted by the solenoid coil on the armature exceeds the sum of the forces acting in the closing direction, i.e., the spring closing force exerted by the first resetting spring and the hydraulic forces of the fuel, which is under pressure. This is disadvantageous inasmuch as, when the current exciting the solenoid coil is switched on, the magnetic force, due to the self inductance of the solenoid coil and the eddy currents arising, has not yet reached its final value. Therefore, at the beginning of the opening stroke, the valve needle and the valve-closure member are accelerated at a reduced force. This leads to an opening time that is not satisfactory for all application cases.
In the closing motion, the known one-part armature sticks for a relatively long time to the magnetized internal pole and detaches itself due to the residual magnetization only after a relatively long time. This leads to relatively long closing times.
In contrast, the fuel injector according to the present invention having the features of the main claim has the advantage that the opening and closing times of the fuel injector attained by the two-part armature are reduced, thus resulting in a greater metering precision for the fuel. This is achieved by the armature detaching very rapidly from the internal pole, in comparison to a one-part armature. The resetting spring, having a large spring constant, directly contacts only one of the armature parts and has only to detach this part from the internal pole. Since the contact surface, which this armature part forms along with the internal pole, is significantly smaller than the entire contact surface, which the entire two-part armature forms along with the internal pole, this armature part detaches itself early from the internal pole, so that the closing motion begins early.
In addition, if the coordination of the mass ratio is good, the use of a two-part armature makes possible a debouncing of the system, in that, when the exciting current is switched off, the time difference between the acceleration of the larger armature part and that of the smaller armature part causes the two armature parts to strike each other in opposite directions. This leads to the elimination of the impulse of the slightly rebounding armature part, as a result of which an undesirable further short-term opening of the fuel injector is prevented.
Through the measures indicated in the subclaims, advantageous refinements and improvements of the fuel injector indicated in the main claim are possible.
Also advantageous is a slight, radial bevel or wedge-like shape of the armature end face that strikes the internal pole. As a result of a wedge-shaped surface configuration, the contact surface between the armature and the internal pole is reduced, and therefore the adhesive power acting between the armature and the internal pole is lessened. In this manner, when the magnetic field is reduced, the armature detaches itself more rapidly from the internal pole, as a result of which the valve closing time is shortened.
Especially advantageous is also the application of the prestroke principle. A prestroke gap between the larger armature part and the supporting flange makes possible a preacceleration of the two armature parts, as a result of which there is a starting impulse in the stroke direction. This is advantageous inasmuch as, when the current exciting the solenoid coil is switched on, the magnetic force, due to self inductance and eddy currents, has not yet reached its final value. However, the time that is gained by the prestroke is not sufficient to set up the magnetic field completely. Therefore, at the beginning of the opening stroke, the valve needle and the valve-closure member are accelerated at an unreduced force. This results in short and precise opening and metering times.